There are half a dozen questions here why the iPad doesn't charge (or displays this at least) on most Windows machines. I have one more to add.

Of course my iPad (1st gen) does charge on my MacBook (late 2008) under Mac OS X. However, when I boot Win 7 in Boot Camp, it says "not charging".

As I see, the logic board is capable to deliver sufficient power. Why does this change under Windows? And what could I do about this, since I do need to use Windows sometimes and I'd like to charge my post-PC device then.

Despite the fact that your iPad says it's not charging under Windows, it will in fact charge if the display is off. Check your battery percentage, attach the iPad and turn off the display (standard locking by pressing the power button once); check back in 30 minutes or so and you'll see that you've definitely gained several percent, as much as 10% in that time.

As to why the iPad indicates that it's charging under OS X but not under Windows I cannot say. Fortunately it's lying. :)

If you look at your iPad charger you will see it has 2.5A and a regular USB port can only give you 500mA. So it's quite useless to charge your iPad from computer. If it says 'not charging', then there is not enough voltage (the voltage may get lower when you connect 'hungry' devices). But I can't get why it may be different in OS X and Win.

Macs can supply more then 500ma. "To meet requests for additional power from these Apple peripherals and devices, some Apple computers and displays can provide up to 1100 mA at 5 V through the port to which the Apple peripheral or device is connected." support.apple.com/kb/HT4049
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vcsjonesApr 28 '11 at 18:53

The fastest way to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter. When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain. -apple.com
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hobsApr 28 '11 at 22:28